【正文】 ainly of the contractile protein actin.The Cytoskeleton（細(xì)胞骨架）All eukaryotic cells have a cytoskeleton, which is a convoluted latticework of filaments and tubules that appears to fill all available space in the cell and provides support for various other organelles.葉綠體是最重要的色質(zhì)體，含有與光合作用有關(guān)的葉綠素。There are two types of plastids: leucoplasts, which lack pigments and serve as storage sites for starch, proteins, and oils。另外，植物細(xì)胞中的質(zhì)體在光合作用中利用光能產(chǎn)生碳水化合物，線粒體內(nèi)嵴上提供了很大的表面積并分布著產(chǎn)ATP酶。消化食物微粒和降解損傷的細(xì)胞殘片。動(dòng)物中的液泡起吞噬和胞飲作用。Vacuoles in animal cells carry out phagocytosis (the intake of particulate matter) and pinocytosis (vacuolar drinking).細(xì)胞中的液泡好象是中空的，但實(shí)際上充滿了液體和可溶分子。The most prominent vacuoles appear in plant cells and serve as water reservoirs and storage sites for sugars and other molecules.Vacuoles in cells appear to be hollow sacs but are actually filled with fluid and soluble molecules.Within the Golgi plex molecules are modified and packaged for export out of the cell or for delivery else where in the cytoplasm.運(yùn)輸小泡能夠?qū)⒖蛇\(yùn)輸分子從內(nèi)質(zhì)網(wǎng)運(yùn)輸?shù)礁郀柣鶑?fù)合體上。Transport vesicles may carry exportable molecules from the endoplasmic reticulum to another membranous organelle, the Golgi plex.Both types of endoplasmic reticulum serve as partments within the cell where specific products can be isolated and subsequently shunted to particular areas in or outside the cell.光滑內(nèi)質(zhì)網(wǎng)上無(wú)核糖體，主要作用是脂肪和類固醇的合成以及細(xì)胞內(nèi)有毒物質(zhì)的氧化。 it is active in the synthesis of fats and steroids and in the oxidation of toxic substances in the cell.粗糙內(nèi)質(zhì)網(wǎng)上分布許多核糖體，也可能提供細(xì)胞分裂后所需的細(xì)胞膜。The RER, which is studded with polysomes, also seems to be the source of the nuclear envelope after a cell divides.內(nèi)質(zhì)網(wǎng)，帶有花邊的生物囊，有管狀，泡狀之分，以及光滑和粗糙面區(qū)別。Both types play roles in the synthesis and transport of proteins.The endoplasmic reticulum, a lacy array of membranous sacs, tubules, and vesicles, may be either rough (RER) or smooth (SER).大多數(shù)細(xì)胞蛋白是由細(xì)胞質(zhì)中核糖體生產(chǎn)。核糖體沿著mRNA移動(dòng)并閱讀遺傳密碼，翻譯成蛋白質(zhì)。Exportable proteins and membrane proteins are usually made in association with the endoplasmic reticulum.核糖體的數(shù)量變化從幾百到幾千，核糖體是氨基酸組裝成蛋白質(zhì)的重要場(chǎng)所。Most cellular proteins are manufactured on ribosomes in the cytoplasm. such a bination is called a polysome.本節(jié)主要介紹核糖體，內(nèi)質(zhì)網(wǎng)，高爾基體系，液泡，溶酶體，線粒體和植物細(xì)胞中的質(zhì)體。核膜貫穿許多小孔，小分子可以自由通過(guò)核膜，而象mRNA和核糖體等大分子必須通過(guò)核孔運(yùn)輸。Small molecules can pass through the nuclear envelope, but larger molecules such as mRNA and ribosomes must enter and exit via the pores.真核細(xì)胞的細(xì)胞核是最大的細(xì)胞器，細(xì)胞核對(duì)染色體組有保護(hù)作用（原核細(xì)胞的遺傳物質(zhì)存在于擬核中）。A poreperforated sac called the nuclear envelope separates the nucleus and its contents from the cytoplasm.The Nucleus: Information Central（細(xì)胞核：信息中心）The eukaryotic cell nucleus is the largest organelle and houses the genetic material (DNA) on chromosomes. (In prokaryotes the hereditary material is found in the nucleoid.) The nucleus also contains one or two organellesthe nucleolithat play a role in cell division.細(xì)胞器懸浮在其中，并由絲狀的細(xì)胞骨架支撐。Dissolved in the cytoplasmic fluid are nutrients, ions, soluble proteins, and other materials needed for cell functioning.生命的大部分特征表現(xiàn)在細(xì)胞質(zhì)的特征上。Organelles are suspended within it, supported by the filamentous network of the cytoskeleton.Much of the mass of a cell consists of this semifluid substance, which is bounded on the outside by the plasma membrane.第一課Cytoplasm: The Dynamic, Mobile Factory細(xì)胞質(zhì)：動(dòng)力工廠Most of the properties we associate with life are properties of the cytoplasm.細(xì)胞質(zhì)大部分由半流體物質(zhì)組成，并由細(xì)胞膜（原生質(zhì)膜）包被。細(xì)胞質(zhì)中溶解了大量的營(yíng)養(yǎng)物質(zhì)，離子，可溶蛋白以及維持細(xì)胞生理需求的其它物質(zhì)。細(xì)胞核含有一或二個(gè)核仁，核仁促進(jìn)細(xì)胞分裂。Organelles: Specialized Work Units（細(xì)胞器：特殊的功能單位）All eukaryotic cells contain most of the various kinds of organelles, and each organelle performs a specialized function in the cell.Organelles described in this section include ribosomes, the endoplasmic reticulum, the Golgi plex, vacuoles, lysosomes, mitochondria, and the plastids of plant cells.所有的真核細(xì)胞都含有多種細(xì)胞器，每個(gè)細(xì)胞器都有其特定功能。The number of ribosomes within a cell may range from a few hundred to many thousands.This quantity reflects the fact that, ribosomes are the sites at which amino acids are assembled into proteins for export or for use in cell processes.A plete ribosome is posed of one larger and one smaller subunit.During protein synthesis the two subunits move along a strand of mRNA, reading the genetic sequence coded in it and translating that sequence into protein.Several ribosomes may bee attached to a single mRNA strand。完整的核糖體由大亞基和小亞基組成。一條mRNA上可能有多個(gè)核糖體，稱多聚核糖體。輸出蛋白和膜蛋白通常與內(nèi)質(zhì)網(wǎng)有關(guān)。兩種都與蛋白質(zhì)的合成和運(yùn)輸有關(guān)。SER lacks polysomes。兩種內(nèi)質(zhì)網(wǎng)合成的產(chǎn)物在其中進(jìn)行分流或運(yùn)輸?shù)郊?xì)胞外。在高爾基復(fù)合體中修飾，包裝后輸出細(xì)胞或傳遞到細(xì)胞質(zhì)中的其他場(chǎng)所。最典型的液泡存在于植物細(xì)胞中，儲(chǔ)備水，糖以及其它分子。A subset of vacuoles are the organelles known as lysosomes, which contain digestive enzymes (packaged in lysosomes in the Golgi plex) that can break down most biological macromolecules.They act to digest food particles and to degrade damaged cell parts.溶酶體是液泡亞單位，含有消化酶，降解大部分生物大分子。Mitochondria are the sites of energyyielding chemical reactions in all cells.In addition, plant cells contain plastids that utilize light energy to manufacture carbohydrates in the process of photosynthesis.It is on the large surface area provided by the inner cristae of mitochondria that ATPgenerating enzymes are located.Mitochondria are selfreplicating, and probably they are the evolutionary descendants of what were once freeliving prokaryotes.線粒體是細(xì)胞中化學(xué)產(chǎn)能的場(chǎng)所。線粒體自我復(fù)制，并且可能是自由生活的原核生物在進(jìn)化中形成的后代。 and chromoplasts, which contain pigments.The most important chromoplasts are chloroplastsorganelles that contain the chlorophyll used in photosynthesis.The internal structure of chloroplasts includes stacks of membranes called grana, which are embedded in a matrix called the stroma.質(zhì)體有兩種類型：白色體，缺乏色素，是淀粉，蛋白質(zhì)和油的儲(chǔ)備場(chǎng)所；色質(zhì)體，含有色素。葉綠體的內(nèi)部結(jié)構(gòu)是由多層膜形成的葉綠體基粒，其中包埋在基質(zhì)中的基粒稱子座。細(xì)胞骨架大部分由微絲組成，微絲主要由可收縮的肌動(dòng)蛋白組成。第二類蛋白是肌球蛋白，它與肌肉細(xì)胞的收縮有關(guān)。細(xì)胞骨架的中間絲提供了細(xì)胞質(zhì)伸縮動(dòng)力。Cellular Movements（細(xì)胞運(yùn)動(dòng)）Although the cytoskeleton provides some stability to cells, its microtubules and filaments and their associated proteins enable cells to move by creeping or gliding.Such